The invention relates to a vessel having an elongated shape, and being moored to the seabed in a substantially fixed orientation.
It is known that elongated vessels, in particular vessels having a length-to-width ratio of 1.5 or less, may be subject to roll instability or Mathieu instability. This roll instability, or parametric resonance causes roll due to heave-induced modulation of the roll hydrostatic stiffness of the system. In particular, the most severe condition would occur when the heave natural period is half the roll natural period.